1. Field of the Invention
The present invention relates to a method for producing a composition containing soluble isoflavones.
2. Description of the Related Art
Soybean isoflavones are a group of compounds having a 3-phenylchromone skeleton that exist in soybeans. Specifically, soybean isoflavones are present in the forms of glucosides, malonyl glucosides, acetyl glucosides or aglycones, etc. The glucosides include daidzin, genistin and glycitin. The malonyl glucosides include 6″-O-malonyldaidzin, 6″-O-malonylgenistin and 6″-O-malonylglycitin. The acetyl glucosides include 6″-O-acetyldaidzin, 6″-O-acetylgenistin and 6″-O-acetylglycitin. The aglycones include daidzein, genistein and glycitein. The general structures and the species of the above compounds can be expressed by formulae (1)-(2) below:
(1) The group of compoundshaving the general skeletonexpressed by formula (1)R1R2R3daidzinHHHgenistinOHHHglycitinHOCH3H6″-O-malonyldaidzinHHCOCH2COOH6″-O-malonylgenistinOHHCOCH2COOH6″-O-malonylglycitinHOCH3COCH2COOH6″-O-acetyldaidzinHHCOCH36″-O-acetylgenistinOHHCOCH36″-O-acetylglycitinHOCH3COCH3
(2) The group of compoundshaving the general skeletonexpressed by formula (2)R4R5daidzeinHHgenisteinOHHglyciteinHOCH3
The above isoflavones are known to have estrogen-like functions and antioxidative functions, etc., and therefore get worldwide attentions as food components effective to prevent cancer and osteoporosis or alleviate the menopausal syndrome. However, isoflavone is difficult to dissolve in water, and its solubility in water is 0.002-0.003 g under 25° C. according to the literature. Consequently, cloudiness or precipitation easily occurs in or after the manufacturing process of food products, especially drink products or dessert products, etc, and the applications of isoflavones are restricted in these fields. Therefore, improvement in the solubility of isoflavones is required.
To solve the above problem, for example, Japanese Patent Application Laid Open No. 09-309902 or 10-298175 discloses a method that includes isoflavone molecules in cyclodextrin to improve its solubility in water. However, since it is necessary to previously refine the isoflavones to certain purity in the method, the operation steps are complicated. Moreover, since cyclodextrin is used when isoflavones are added into drink products, the aroma components like flavors are also included in cyclodextrin together with the isoflavone molecules. Therefore, the balance of aroma is easily broken down, and the commercial product design is difficult accordingly. Moreover, isoflavones cannot be dissolved in water in a high concentration due to the limitation of the solubility of cyclodextrin. On the other hand, Japanese Patent Application Laid Open No. 2000-325043 discloses another method that dissolves isoflavones by heating the compounds together with a solubilizing agent consisting of dehydrated or hydrated propylene glycol and/or octenylsuccinic acid starch in the presence of water. Nevertheless, the food additives like propylene glycol and octenylsuccinic acid starch are not preferable to use in latest years. Moreover, Japanese Patent Application Laid Open No. 2000-327692 discloses a method for improving the solubility of isoflavones in water, which forms α-glycosylisoflavone derivatives utilizing a glycosyltransferase in the presence of an α-glycosylsaccharide compound like dextrin. The α-glycosylisoflavone derivative is formed by attaching a glucose residue to a daidzin or genistin molecule with an α-1,4 bond. However, since one or more equivalents of the α-glycosylsaccharide compound must be used to bond with the isoflavone molecules, the process is complicated. Moreover, the percentage of isoflavones in total solid content is surely lowered in the method, and there is no isoflavone molecule present in its natural state. Furthermore, though the solubility and the long-term stability under low temperature both are important properties when a composition containing isoflavones is added in a liquid food product like a drink product, there is no information about them. Each of the aforementioned methods solubilizes isoflavones either by adding a solubilizing agent or by chemically modifying isoflavone molecules to form soluble derivatives, while there is no easier method suitably used for industrial production that solubilizes insoluble isoflavones in their natural form within a wide pH range.